Voltage controlled frequency circuit



Nov.'2l, 1961 B. STEFANOV 3,010,078

. VOLTAGE CONTROLLED FREQUENCY CIRCUIT Filed Sept. 21, 1959 II l2 IO 2|I l9 I8 l C2 H c, 27

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- E L L 1 v 3 TIME 7 EC EZ' L TIME TIME h: INVENTOR. :T 2 BORIS STEFANOV2?" 3 ATTORNE Z$ This invention relates to a novel electrical circuitfor providing an output signal of frequency which may be modulated by aninput voltage.

A primary object of the invention is to provide an extremely simple andinexpensive electrical circuit for converting an amplitude modulatedinput voltage to a frequency modulated output signal in which suchfrequency over the entire modulation range is directly proportional tothe particular input voltage level.

Another object is to provide a circuit meeting the foregoing objectswhich avoids the use of inductances in the frequency control portion ofthe circuit and thereby avoids manufacturing inaccuracies andtemperature compensation problems associated with inductance coils.

Briefly, the invention contemplates an electrical circuit including astorage condenser in conjunction with means for repeatedly charging thecondenser to a fixed voltage value in response to completion of thedischarge thereof. The fixed voltage value to which the condenser ischarged and the zero ground potential at complete discharge arecarefully regulated so that the actual discharging takes place betweenfixed constant voltage limits. A discharge current control means isincorporated in the discharge path of the condenser and is arranged tovary the rate of discharge in accordance with the voltage value of aninput voltage. By this arrangement, the rate of discharging of thecondenser and thus the frequency of the output signals derived from thecircuit may be accurately controlled by the particular value of theinput voltage.

A better understanding of the invention will be had by referring to oneembodiment thereof as illustrated in the accompanying drawings, inwhich:

FIGURE 1 is a schematic diagram of the voltage controlled frequencycircuit; and,

FIGURE 2 illustrates a series of wave forms useful in explaining theoperation of the circuit of FIGURE 1.

Referring first to FIGURE 1, there are shown first and second transistorelements and 11 having emitter terminals 12 and 13 connected together toa common input lead 14. A fixed voltage is maintained on the lead 14 bya regulator comprising resistance 15 and a zener diode 16 connectedbetween ground and the junction point of the resistance 15 and lead 14.Input voltage from any suitable source is applied to the input terminal17. With this arrangement, the emitter terminals are subject to aconstant fixed voltage determined by the zener voltage E of the diode16.

The first and second transistors 1i and 11 include first and secondcollector terminals 18 and 19 respectively. Also included are first andsecond storage condensers C and C The first storage condenser C has oneside connected to the collector lead 19 of the second transistor '11 andthe second storage condenser C has one side connected to the firstcollector lead 18 of the first transistor 10. The other sides of thecondensers C and C connect respectively to the base leads 2t and 21 forthe first and second transistors as shown. Output resistances 22 and 23in turn connect from the collector lead sides of the condensers C and Cto ground 24.

The foregoing arrangement constitutes a multi-vibrator circuit fromwhich an output signal in the general form of a square Wave may be takenfrom across either one States Patent Q a 3,016,078 Patented Nov. 21,1961 of the resistances 22 or 23. In the embodiment shown forillustrative purposes, output leads 25 and 26 connect across theresistance 22 to terminate in output terminals 27 and 28. The outputvoltage is indicated by E The current discharge paths for the condensersC and C include, in series, discharge current control means in the formof first and second current control transistors 29 and 30. As shown,collector terminals 31 and 32 connect to the base terminal loads 20 and21 and the emitter terminals 33 and 34 connect through resistances 35and 36 and a common variable resistance 37 to a negative voltage inputterminal 38. The base leads 39 and 40 of the transistors 29 and 30 aretied together. An input voltage E is applied between the base terminalsand ground through suitable input terminals 42. The particular value ofthe input voltage E will thus determine the voltage at the baseterminals 39 and 4-0. This base voltage in turn will control the currentpassed by the transistors 29 and *30.

The operation of the circuit is as follows. If I represents thedischarge current in the base lead 20* for the condensers C and I thedischarge current for the condenser C the multi-vibrator action willresult in the presence of I and I at alternate periods of time. Sincethe circuit is entirely symmetrical, a description of ,the action takingplace with respect to the current I and condenser C will suflice, itbeing understood that the current I and condenser C function in asimilar manner. Since E is fixed by action of the zener diode 16, thecondenser C is charged to an initial fixed voltage E If T represents /2the period of the contemplated output wave form of the multi-vibrator,the following relationship exists:

Since 2T will equal the period of the output wave form, its frequency fis given Therefore, the current I can be expressed by:

where K is a second constant and Thus,

of the property E is the input voltage.

J2 f l i or in other words 7 is directly proportional to the inputvoltage which is the desired relationship to be effected by the circuit.

Stated differently, by changing the input voltage level E thedischarging current through the transistor 29 can be changed. Changingthe value of the discharging current changes. the rate of dischargewhich in turn will change the period of the output wave form and thusthe frequency.

As an example, FIGURE 2 shows three different levels of input voltages Eindicated at E E and E Considering first the input voltage level E therewill be established a constant current proportional to E passing throughtransistor 29 for effecting the discharge of the condenser C Theresulting voltage E across the condenser during discharge is indicatedby the second curve E in FIG- URE 2. It will be noted that thisdischarge takes place between the fixed voltage E to which the condenseris initially charged and zero potential. Moreover, the slope of thedischarge cu-rve depends on the value of the discharge current in turncontrolled by the level of the in put voltage. 7

When the condenser C is completely discharged, the circuit is switchedby action at the base 2% of the transistor I turning on the transistor10 to charge the condenser C to the input voltage E The condenser C thendischarges during which time the voltage across the condenser C is heldat zero as soon as transistor 11 is cut oif. When the discharge of C iscomplete, the transistor 11 is switched on and the transistor 10switched oif and the cycle repeated. Since the discharge current 1 isheld constant, the voltage across the output resistances .22 and 23 willbe constant during the discharge portions of the cycle. There thusresults a square wave of frequency h as shown at E in FIGURE 2.

If now the input voltage level is changed to a higher value E as shownin the middle portion of FIGURE 2, a higher fixed or constant current Ipasses through the transistor 29 thereby resulting in an increased rateof discharge. Since the voltage limits between Which the discharge takesplace across the condenser C are always the same value, the discharge iscompleted sooner and the switching frequency is correspondinglyincreased. In fact, if the input voltage level E is doubled, asindicated by the level E in FIGURE 2, the frequency of the output signalwill similarly be doubled as indicated at f in the lower central plot inFIGURE 2.

Finally, if the input voltage E is now decreased to an intermediatelevel between E and E the frequency i of the output signal willsimilarly assume an intermediate frequency value.

As mentioned, the wave forms across the condenser C and outputresistance 21 are identical to those shown for the voltages across thecondenser C and output resistance except that they are shifted in timephase 180- degrees.

From the foregoing description, it will be evident that there isprovided an extremely simple method for providing a frequency modulatedsignal in which the modulation is directly proportional to themodulation of an input voltage. This proportionality or linearity is,moreover, substantially perfect over a very wide range of input voltagevariations, as a consequence of employing only con- 4 densers andresistances which are relatively easy to compensate for temperaturevariations. The use of inductances, which are difficult to manufacturewith precision and compensate, is avoided by the present circuit.

Minor modifications falling within the scope and spirit of thisinvention will occur to those skilled in the art. The voltage controlledfrequency circuit is therefore not to be thought of as limited to theparticular embodiment set forth merely for illustrative purposes. i

What is claimed is:

l. A voltage controlled frequency circuit for providing an output waveform of frequency directly proportional to an input voltage value,comprising, in combination: first and second transistors having emitter,collector, and base terminals; a first storage condenser connectedbetween the collector terminal of said second transistor and the baseterminal of said first transistor; a second storage condenser connectedbetween the collector terminal of said first transistor and the baseterminal of said second transistor; a fixed voltage source connected toboth emitter terminals of said first and second transistors; first andsecond output resistances connected respectively between said collectorterminals and ground; first and second discharge paths for said firstand second condensers respectively; and first and second dischargecurrent control means in said first and second discharge pathsrespectively and responsive to said input voltage value for providingdischarge currents proportional to said input voltage value.

2. The subject matter of claim '1, in which said first and seconddischarge current control means comprise transistors having base,collector, and emitter terminals, said collector and emitter terminalsbeing connected in series respectively in said first and seconddischarge paths, and said base terminals being connected to receive saidinput voltage.

References Qited in the file of this patent UNITED STATES PATENTS2,564,687 Guenther Aug. 21, 1951 2,841,712 Hoge et al July 1, 19582,894,215 Toy". July 7, 1959 2,960,668 Young Nov. 15, 1960

